Sandwich panel and method for producing such a panel

ABSTRACT

A panel possessing mutually averted panel surfaces and a periphery which delimits the panel surfaces is made up of a series of elongated core elements positioned parallel to one another, as well as material strips which are affixed to the core elements and which each extend between respectively two neighboring core elements and along both panel surfaces. Such a panel can expediently be constructed by placing a number of core elements and strips of flexible material side by side, or by stacking these one upon the other.

The invention relates to sandwich panels of fiber-reinforced plasticsmaterials, which are particularly suitable for relatively high loads. Byway of example, sandwich panels are cited which can be used for the deckof a bridge or as a self-supporting bridge construction. In suchapplications, the panel must be able to withstand the heavy loads ofroad traffic. These loads take different forms. It is firstly a matterof the total weight of traffic present on a bridge, which manifestsitself in bending and torsion of the panel between the supports thereof.In addition, account must be taken of local loads, such as caused by thewheel load of heavy freight traffic, to which the surface of thesandwich panel, in particular, is exposed.

Such local loads will lead to deformation of the bridge deck. If freightfalls from a truck, for example, then the local load can give rise topermanent damage to the surface of the sandwich panel. In thisconnection, falling-off freight such as concrete parts, rail tracks andthe like can be thought of. Although a bridge deck made up of steelpanels on steel girders undergoes a permanent deformation, manifested inindentations or perforations, as a result of the high local load whichis hereupon generated, the total integrity of the construction inquestion is not thereby affected. Hence, the load-bearing capacity ofthe bridge itself is not endangered.

For various reasons, the production of bridge decks from materials otherthan steel or concrete is sought. One example of such an alternative isprovided by the bridge decks produced from fiber-reinforced plasticslaminates. Such a bridge deck can be produced with suitable templates ina variety of different forms. By virtue of the strong fibers which areavailable, the total resistance of such a bridge deck with regard togeneral bending and torsion is excellent, such that panels produced inthis way could withstand the heaviest loads of heavy freight traffic. Adrawback of fiber-reinforced plastic laminates is however theirrelatively low resistance to local loads or point loads, that is to saya static load or an impact load. It is precisely such loads which cancause damage in the panel, resulting in a delamination or detachment ofthe outer skin from the sandwich panel which gets progressively worseunder the influence of the passing traffic.

A panel having mutually averted panel surfaces and a periphery whichdelimits the panel surfaces, comprising a series of parallellypositioned, elongated core elements, as well as material strips whichare affixed to the core elements and which each extend betweenrespectively two neighboring core elements and along both panelsurfaces, is known from WO-A-9009880. The sole object which is aimed forwith this known construction is the enlargement of the contact surfacebetween the web plates and the skin.

The object of the invention is to provide a sandwich panel which is madeup of fiber-reinforced plastic components and which nevertheless meetsthe requirements with regard to point and impact loads. This object isachieved by virtue of the fact that those portions of the materialstrips which extend along the panel surfaces extend over a plurality ofcore elements. By a series of parallelly positioned core elements ismeant a multiplicity of core elements which are located side by sideand/or one upon the other, such that the product thus formed has a widthand/or height amounting to a multiple of the width and/or height of acore element.

During the construction, the cores and the material strips can beloosely stacked. A glue, or a mechanical joint, can here possibly beused to obtain a temporary connection. In the finished construction, thematerial strips are affixed to the core elements, that is to sayfollowing injection and setting of the resin.

The panel according to the invention consists of a large number of coreelements extending parallelly alongside one another, which are mutuallyconnected by the material strips which extend over, under and betweenthe core elements. In this context, it is important that the panelsurfaces (the skins) and surfaces extending between respectively twoneighboring core elements (the web plates) of the sandwich panel aremade up of the same material strips. In conventional sandwich panels, adistinction can always at least partially be drawn between materialstrips used for the web plates, on the one hand, and material stripsused for the skins, on the other hand. Firstly, such a constructionaccording to the invention ensures a particularly robust mutualconnection of the core elements and skins. Secondly, the skins thusformed and the ribs formed between the core elements are integrallyconnected to one another. For the ribs merge directly into the skins,which skins consist of those parts of the material strips, stackedobliquely one above the other, which extend over the panel surfaces.There is hence no danger of delamination between the ribs and the skins.Consequently, the panel which is thus obtained can comfortably meet therequirements with regard to bending and torsion and impact load. Thecore elements which are here used can possess different cross sectionalforms, such as square, rectangular, but triangular cross sectional formsare also possible. In this last variant, including viewed in thetransverse direction of the sandwich panel, the forces can be readilytransmitted. Furthermore, the core elements can be of tubularconstruction, though solid core elements may also be used. The panelsurfaces of the panel thus obtained are formed by the core elements, aswell as by those portions of the various material strips which extendthereover. These material strips are stacked one upon the other; thenumber in the stack can be varied by adjusting the width, that is to saythe dimension of material strips transversely to the longitudinaldirection of the core elements. If now a large local load or point loadis applied to such a sandwich panel, such as caused, for example, by theabove-described falling-off freight, the surface of the sandwich panelcan get damaged. This implies that locally the material strips aredamaged and that possibly one or some of the core elements are likewisedamaged.

The important advantage of the sandwich panel according to the inventionis, however, that this damage has a very local character and does notdisplay a tendency to spread over the rest of the construction. Eventhough one or some material strips and core elements are locallydamaged, because the rest of the material strips and core elements, thatis to say in the transverse direction beyond the damage, remains intact,those parts of the material strips which are found there are capable ofmaintaining the integrity of the panel in the direction of overstress.

The material strips can extend in a variety of ways relative to the coreelements. According to a first option, the material strips can eachcomprise a portion located between two neighboring core elements, aportion extending over one neighboring element and along one panelsurface, as well as a portion extending over the other neighboring coreelement and along the other panel surface. In such a case, the materialstrips possess essentially a Z-shape.

According to another option, the material strips can each extend overone and the same neighboring element and along both panel surfaces. Inthis case, the material strips possess essentially tilted U-shape.

As already stated, those portions of the material strips which arelocated along the panel surfaces extend over a plurality of coreelements. Preferably, the material strips forming the panel surfacedescribe an angle of inclination of 20 degrees or less relative to thetransverse direction of the panel.

According to one possible variant, material strips can comprise aportion located between two neighboring core elements and extending onlyover a portion of these core elements, as well as a portion extendingonly along a panel surface.

According to yet another variant, core elements close to both panelsurfaces can be provided with material strips, which comprise a portionlocated between two neighboring core elements and extending only over aportion of these core elements, as well as a portion extending onlyalong a panel surface.

Furthermore, those portions of two material strips which extend betweenrespectively two neighboring core elements are overlapped by aconnecting strip located between these core elements. The inventionfurther relates to a method for producing a panel, such as for a bridgedeck, comprising the following steps:

-   -   the provision of a core element,    -   the covering of this core element with a strip of impregnable        flexible material which continues transversely to the        longitudinal direction of the core element to beyond this core        element,    -   the provision of a following core element alongside the        preceding covered core element and on that portion of the strip        of flexible material covering this preceding core element which        is continued beyond the preceding core element,    -   the covering of the following core element with a following        strip of flexible material which continues transversely to the        longitudinal direction of the following core element to beyond        this following core element,    -   the at least single repetition of the step of providing a        following core element, the step of placing this following core        element alongside a preceding covered core element and on that        portion of the preceding strip of flexible material covering the        preceding core element which is continued beyond the preceding        core element, as well as the step of covering the following core        element with a following strip of flexible material which        continues transversely to the longitudinal direction of the        following core element to beyond this following core element,    -   the impregnation of the strips of material with a hardenable        fluid,    -   the hardening of the impregnated strips of material.

This method is characterized by the fitting of the following strip offlexible material over the top face of a following core element, as wellas over the preceding strip of flexible material extending over the topface of a preceding core element.

In the method according to the invention, a panel is obtained whichconsists of a large number of core elements extending parallellyalongside one another, which are mutually connected by the strips offlexible material folded essentially into a Z-shape, which extend over,under and between the core elements.

The method according to the invention can be implemented in a variety ofways; preferably, however, a template is used herein. In this case, themethod comprises the following steps:

-   -   the positioning of a core element in a template,    -   the fitting of a strip of impregnable flexible material over the        top face and a side face of the core element, as well as over        that bottom portion of the template which adjoins this side        face,    -   the positioning of a following core element in the template        against the flexible material which covers said side face of        said core element and on the flexible material which covers said        bottom portion,    -   the fitting of a following strip of flexible material over the        top face and a side face of the following core element, as well        as over that bottom portion of the template which adjoins this        side face,    -   the repetition of the positioning of a following core element        and a following strip of flexible material until the desired        number of core elements alongside one another is obtained,    -   the impregnation of the strips of flexible material with a        hardenable fluid,    -   the hardening of the hardenable fluid, with formation of the        panel.

The shape of the template is here pre-chosen such that the desired shapeof the panel is obtained, for example with a slight curvature in thelongitudinal direction, in the transverse direction, etc.

As stated above, the panel surfaces of the panel are formed by the coreelements and by the strips of flexible material extending thereover. Thetotal thickness of these surfaces of the panels can be enlarged bymaking the various strips of flexible material extend more or less farover one another. If, for example, each strip of flexible materialextends over three or more core elements, a triple-layered ormultilayered surface is also formed. As already stated, this is obtainedby the step of:

-   -   fitting the following strip of flexible material over the top        face of a following core element, as well as over the preceding        strip of flexible material extending over the top face of a        preceding core element.

On the bottom side of the panel also, the thickness of the surface canthus be influenced by the step of:

-   -   fitting the following strip of flexible material over the        preceding strip of flexible material extending from a preceding        core element under the bottom face and past the side face of the        following core element.

In a known manner, the strength and stiffness characteristics of thepanel are influenced by the nature of the strips of flexible material.In particular, the method according to the invention can here comprisethe steps of using strips of flexible material which comprise a fabric,and of aligning the threads in the longitudinal direction of the coreelements. In the same way, the method according to the invention cancomprise the steps of using strips of flexible material which comprise afabric, and of aligning the threads of the fabric in the transversedirection of the core elements.

A variant of the method for producing a panel comprises the followingsteps:

-   -   the provision of a core element,    -   the covering of this core element with a strip of impregnable        flexible material which continues transversely to the        longitudinal direction of the core element to beyond this core        element,    -   the provision of a following core element on the preceding        covered core element and on the preceding strip of flexible        material,    -   the covering of the following core element with a following        strip of flexible material which continues transversely to the        longitudinal direction of this following core element to beyond        this following core element,    -   the at least single repetition of the step of providing a        following core element, the step of placing this following core        element on a preceding core element and a preceding strip of        flexible material covering the preceding core element, as well        as the step of covering the following core element with a        following strip of flexible material which continues        transversely to the longitudinal direction of the following core        element to beyond this following core element, with formation of        a stack of core elements and intervening parts of the strips of        flexible material,    -   the abutment of those parts of the strips of flexible material        which extend to beyond the stack against this stack,    -   the impregnation of the strips of material with a hardenable        fluid,    -   the hardening of the impregnated strips of material into an end        product.

This method is characterized by the fitting of the following strip offlexible material over a following core element, as well as over thepreceding strip of flexible material extending over a preceding coreelement.

In particular, this method can be implemented with the following steps:

-   -   the provision of a template, the internal shape of which        conforms to the contour of the end product to be produced,    -   the placement of the template over the stack and those parts of        the strips of flexible material which extend relative to this        stack,    -   the hardening of the impregnated strips of material while the        template is in position over the stack.

The invention will be explained in greater detail below with referenceto the illustrative embodiments represented in the figures.

FIG. 1 shows the manufacture of a first embodiment.

FIG. 2 shows a second embodiment.

FIG. 3 shows a third embodiment.

FIGS. 4-6 show the steps in the manufacture of a second embodiment.

FIGS. 7-10 show further embodiments.

FIG. 1 shows a portion of a template 4, having a form face 3. Onto thistemplate is laid a first core element 1, which is tubular and consistsof the top wall 5, the right-hand side wall 6, the bottom wall 7 and theleft-hand side wall 8. The core element 1 is covered with a strip ofimpregnable flexible material 2. This strip of flexible material 2 hasan uppermost portion 9 which is laid over the top wall 5, a centermostportion 10 which is laid against the right-hand side wall 6, and alowermost portion 11 which is laid over the floor 3 of the template 4.The strip of flexible material 2 has thereby acquired a Z-shape.

The above-stated steps of putting a core element in place and laying ofa strip of flexible material thereover are then repeated a number oftimes. Thus the following core element 1′ is placed parallel to andalongside the preceding core element 1. The left-hand side wall 8′ ofthe following core element 1′ is here placed against that portion 10 ofthe strip of flexible material 2 which covers the right-hand side wall 6of the core element 1. The bottom wall 7′ of the following core element1′ is placed on that portion 11 of the strip of flexible material 2which extends over the floor 3 of the template 4.

Over the following core element 1′ there is then placed a followingstrip of flexible material 2′. The portion 9′ thereof comes to lie uponthe top wall 5′ of the following core element 1′. It is also the casethat this portion 9′ of the following strip of flexible material 2′ canpossess such a dimension in the transverse direction to the coreelements that this also covers that portion 9 of the strip of flexiblematerial 2 which covers the core element 1. Depending on the particulardimension, this portion 9′ can therefore extend over a plurality ofpreceding core elements and strips of flexible material.

The portion 10′ of the following strip of flexible material 2′ is laidover the right-hand side wall 6′ of the following core element 1′ and,finally, the portion 11′ of the following strip of flexible material 2′is laid over the floor 3 of the template 4. This portion 11′ of thefollowing strip of flexible material 2′ can here also cover a bit of theportion 11 of the previous strip of flexible material 2, which portion11 had already been fitted on the floor 3 of the template 4. Dependingon the dimension in the transverse direction relative to the coreelements of this portion 11 of the strip of flexible material 2, theportion 11 can extend under a plurality of following core elements 1″,etc.

In this way, a plurality of core elements 1′, 1″, etc. placed side byside are respectively covered by respective strips of flexible material2′, 2″, etc.

FIG. 2 shows an alternative embodiment, wherein the uppermost portion 9of the strip of flexible material is laid over the core element 1, thecentermost portion 10 extends over the left-hand side wall 8, and thelowermost portion 11 extends under the bottom wall 7 of this same coreelement 1. The strips of flexible material in these are thus shapedaccording to a tilted U.

FIG. 3 shows that the core elements 1 can also possess a triangularshape.

FIGS. 4-6 show a possible way of producing the embodiment of FIG. 2.Here, a first core element 1 is first put in place, over the top face 5of which a strip of flexible material 1 is laid. On this a followingcore element 1′ is placed, with thereover a following strip of flexiblematerial 2′. These actions are repeated until a stack 12 of the desiredheight is obtained. This stack is formed by all the core elements 1, 1′,1″, . . . and the intervening parts 10, 10′, 10″ . . . of the strips offlexible material 2, 2′, 2″, . . . . The parts 9, 9′, 9″, . . . and 11,11′, 11″, . . . still extend freely beyond the stack 12.

Over this stack is then placed the template 13, the internal contour ofwhich conforms to the shape of the end product which is ultimately to beformed. When the template is pushed onto the stack 12, those parts 9,9′, 9″, . . . and 11, 11′, 11″, . . . of the strips of flexible material2, 2′, 2″, . . . which are located beyond the stack 12 are shaped andpressed against the stack 12. Next, the strips of flexible material areimpregnated; following hardening of these strips, the end product 14 isobtained.

In the variant of FIG. 7, material strips 2 are used possessing a singleportion 9, 11 which covers a top wall 5 and bottom wall 6 of the coreelements 1, as well as a centermost portion 10 which is located betweenthe core elements 1 and extends only over a portion of these coreelements. These centermost portions 10 are connected to each other bythe connecting strips 15.

The variant of FIG. 8 is broadly consistent with that of FIG. 7, thoughthe centermost portion 10 is now folded in two and folded back. Theconnecting strips can in this case be present.

In the variant of FIG. 9, material strips 2 are used having a singleportion 9, 11, which is located along a top wall 5 and bottom wall 6respectively of the core elements and extends over a plurality of coreelements. The middle portion 10 extends over the entire height betweenthe core elements, while, on the rim thereof opposite the rim where thesingle portion 19 is, a small bent-over portion is present, whichextends only over a part of a core element.

In the variant of FIG. 10, the middle portions 10 extend over virtuallythe entire height between the core elements 1, while the material strips2 possess only a single portion 9 and 11 respectively, which extendsover the top face 5 and bottom face 6 respectively.

LIST OF REFERENCE SYMBOLS

-   1, 1′, 1″ . . . core element-   2, 2′, 2″ . . . flexible strip-shaped material-   3 floor of template-   4 template-   5 top wall of core element-   6 right-hand side wall of core element-   7 top wall of core element-   8 left-hand side wall of core element-   9, 9′, 9″ . . . uppermost portion of flexible strip-shaped material-   10, 10′, 10″ . . . centermost portion of flexible strip-shaped    material-   11, 11′, 11″ . . . lowermost portion of flexible strip-shaped    material-   12 stack-   13 template-   14 end product-   15 connecting strip

1. A panel having mutually averted panel surfaces and a periphery whichdelimits the panel surfaces, comprising a series of parallellypositioned, elongated core elements (1, 1′, 1″, . . . ), as well asmaterial strips (2, 2′, 2″, . . . ) which each comprise a portionlocated between respectively two neighboring core elements and at leastone portion extending along one of the panel surfaces, characterized inthat those portions of the material strips which extend along the panelsurfaces extend over a plurality of core elements.
 2. The panel asclaimed in claim 1, wherein material strips comprise a portion extendingover one neighboring element and along one panel surface, as well as aportion extending over the other neighboring core element and along theother panel surface.
 3. The panel as claimed in claim 1, whereinmaterial strips extend over one and the same neighboring element andalong both panel surfaces.
 4. The panel as claimed in claim 1, whereinmaterial strips comprise a portion located between two neighboring coreelements and extending only over a portion of these core elements, aswell as a portion extending only along a panel surface.
 5. The panel asclaimed in claim 4, wherein core elements close to both panel surfacesare provided with material strips, which comprise a portion locatedbetween two neighboring core elements and extending only over a portionof these core elements, as well as a portion extending only along apanel surface.
 6. The panel as claimed in claim 5, wherein thoseportions of two material strips which extend between respectively twoneighboring core elements are overlapped by a connecting strip locatedbetween these core elements.
 7. The panel as claimed in claim 1, whereinthe core elements possess a square or rectangular cross section.
 8. Thepanel as claimed in claim 1, wherein the core elements possess atriangular cross section.
 9. The panel as claimed in claim 1, whereinthe core elements contain material strips, such as in the form of asinusoidal corrugated plate or an omega-shaped profiled plate.
 10. Thepanel as claimed in claim 1, wherein the material strips are affixed toeach other with a hardened impregnating agent.
 11. A method forproducing a panel as claimed in claim 2, comprising the following steps:the provision of a core element (1), the covering of this core element(1) with a strip of impregnable flexible material (2) which continuestransversely to the longitudinal direction of the core element (1) tobeyond this core element (1), the provision of a following core element(1′) alongside the preceding covered core element (1) and on thatportion (11) of the strip of flexible material (2) covering thispreceding core element (1) which is continued beyond the preceding coreelement (1), the covering of the following core element (1′) with afollowing strip of flexible material (2′) which continues transverselyto the longitudinal direction of the following core element (1′) tobeyond this following core element (1′), the at least single repetitionof the step of providing a following core element (1″), the step ofplacing this following core element (1″) alongside a preceding coveredcore element (1′) and on that portion (11′) of the preceding strip offlexible material (2′) covering the preceding core element (1′) which iscontinued beyond the preceding core element (1′), as well as the step ofcovering the following core element (1″) with a following strip offlexible material (2″) which continues transversely to the longitudinaldirection of the following core element (1″) to beyond this followingcore element (1″), the impregnation of the strips of material (2, 2′,2″, . . . ) with a hardenable fluid, the hardening of the impregnatedstrips of material (2, 2′, 2″, . . . ), characterized by: the fitting ofthe following strip of flexible material (2′) over the top face (5′) ofa following core element (1′), as well as over the preceding strip offlexible material (2) extending over the top face (5) of a precedingcore element (1).
 12. The method for producing a panel as claimed inclaim 11, comprising the following steps: the positioning of a coreelement (1) in a template (4), the fitting of a strip of impregnableflexible material (2) over the top face (5) and a side face (6) of thecore element (1), as well as over that bottom portion (3) of thetemplate (4) which adjoins this side face (6), the positioning of afollowing core element (1′) in the template (4) against the flexiblematerial (2, 10) which covers said side face (6) of said core element(1) and on the flexible material (2, 11) which covers said bottomportion (7), the fitting of a following strip of flexible material (2′)over the top face (5′) and a side face (6′) of the following coreelement (1′), as well as over that bottom portion (3) of the template(4) which adjoins this side face (6′), the repetition of the positioningof a following core element (1″) and a following strip of flexiblematerial (2″) until the desired number of core elements (1, 1′, 1″, . .. ) alongside one another is obtained, the impregnation of the strips offlexible material (2, 2′, 2″, . . . ) with a hardenable fluid, thehardening of the hardenable fluid, with formation of the plate girder.13. The method as claimed in claim 11, comprising the step of: fittingthe following strip of flexible material (2′) over the preceding stripof flexible material (2) extending from a preceding core element (1)under the bottom face (8′) and past the side face (6′) of the followingcore element (1′).
 14. The method as claimed in claim 11, comprising thestep of using tubular core elements (1).
 15. The method as claimed inclaim 11, comprising the steps of using strips of flexible materialwhich comprise a fabric, and of aligning the threads in the longitudinaldirection of the core elements.
 16. The method as claimed in claim 11,comprising the steps of using strips of flexible material which comprisea fabric, and of aligning the threads of the fabric in the transversedirection of the core elements.
 17. The method as claimed in claim 11,comprising the step of impregnating the flexible material under vacuum.18. The method as claimed in claim 11, comprising the following steps:the fitting of a strip of flexible material, directly following thefitting of this strip of flexible material, the impregnation thereof,the subsequent fitting of a following core element, the fitting of afollowing strip of flexible material, directly following the fitting ofthis strip of flexible material, the impregnation thereof.
 19. A methodfor producing a panel as claimed in claim 1, comprising the followingsteps: the provision of a core element (1), the covering of this coreelement (1) with a strip of impregnable flexible material (2) whichcontinues transversely to the longitudinal direction of the core element(1) to beyond this core element (1), the provision of a following coreelement (1′) on the preceding covered core element (1) and on thepreceding strip of flexible material (2), the covering of the followingcore element (1′) with a following strip of flexible material (2′) whichcontinues transversely to the longitudinal direction of this followingcore element (1′) to beyond this following core element (1′), the atleast single repetition of the step of providing a following coreelement (1″), the step of placing this following core element (1″) on apreceding core element and a preceding strip of flexible material (2′)covering the preceding core element (1′), as well as the step ofcovering the following core element (1″) with a following strip offlexible material (2″) which continues transversely to the longitudinaldirection of the following core element (1″) to beyond this followingcore element (1″), with formation of a stack (12) of core elements (1,1′, 1″, . . . ) and intervening parts (10, 10′, 10″, . . . ) of thestrips of flexible material (2, 2′, 2″, . . . ), the abutment of thoseparts (9, 9′, 9″, . . . , 11, 11′, 11″, . . . ) of the strips offlexible material (2, 2′, 2″, . . . ) which extend to beyond the stack(12) against this stack (12), the impregnation of the strips of material(2, 2′, 2″, . . . ) with a hardenable fluid, the hardening of theimpregnated strips of material (2, 2′, 2″, . . . ) into an end product(14), characterized by: the fitting of the following strip of flexiblematerial (2′) over a following core element (1′), as well as over thepreceding strip of flexible material (2) extending over a preceding coreelement (1).
 20. The method as claimed in claim 19, comprising thefollowing steps: the provision of a template (13), the internal shape ofwhich conforms to the contour of the end product (14) to be produced,the placement of the template (13) over the stack (12) and those parts(9, 9′, 9″, . . . , 11, 11′, 11″, . . . ) of the strips of flexiblematerial (2, 2′, 2″, . . . ) which extend relative to this stack (12),the hardening of the impregnated strips of material (2, 2′, 2″, . . . )while the template (13) is in position over the stack (12).